1. Field of the Invention
The present invention relates to a method for sealing a vacuum double wall container made of metal, and additionally to an associated sealed container construction.
2. Description of the Conventional Art
In general, a double wall container, made of metal isolated thermally, known as a Dewar's bottle or a vacuum container includes an inner container and an outer container defining a space between double walls, and a vacuum portion producing a wall isolated thermally, wherein the vacuum portion exhausts inner air and seals an exhaust portion so that an insulating layer is formed.
A method for manufacturing a vacuum double wall container made of metal as described hereinabove generally includes the steps of forming a double wall container by joining the inner container and the outer container, piercing an exhaust hole on an appropriate position of a bottom face of the outer container, incinerating impurities, such as oil or the like adhered to a peripheral wall of the vacuum double wall container made of metal, in a vacuum heating furnace, evacuating a space formed between the inner container and the outer container, and sealing the exhaust hole. The sealing of the exhaust hole is a very significant factor having an influence upon the degree of vacuum of the double wall container.
A method for sealing the exhaust hole after evacuating the space between the walls of the double wall container has been proposed, which is disclosed in Japanese Laid-open Patent Publication No. 6-169850.
A method for manufacturing a double wall container according to the Laid-open Patent Publication No. 6-169850 includes the steps of joining the inner container and the outer container to produce the interwall space piercing the exhaust hole on the bottom face of the outer container, putting a solid material capable of fusing at a low temperature in the exhaust hole, heating the solid material in a vacuum heating furnace, evacuating the space portion by exhausting air therein, and sealing the exhaust hole by melting the material.
However, the method for sealing the exhaust hole on the vacuum double wall container by melting the material as described hereinabove has problems in that a composition of the material causes a chemical action, which necessarily generates an expansion phenomenon producing gas, which lowers a density of the material melted by loosening a particle structure of the material and deteriorates the degree of vacuum by weakening air tightness.
The method for sealing the exhaust hole as described hereinabove has further problems in that although it has economy of power consumption by a fusion process at the low temperature, the degree of vacuum of the double wall container becomes low, which makes adiabatic efficiency worse and deteriorates heat insulating ability, since the vacuum double wall container made of metal is evacuated at a low degree of vacuum.
Another method for sealing the exhaust hole is disclosed in Japanese Laid-open Utility Publication No. 3-119342, where in joining the inner container and the outer container at respective mouths and making a void portion between the inner container and the outer container a vacuum insulating layer, the method includes the steps of forming a recessed part in the outer container, piercing the exhaust hole for evacuating the void portion in the recessed part, and sealing the exhaust hole by putting a brazing material in the exhaust hole.
However, the method as described hereinabove has problems in that gas remaining in the space portion does not exhaust completely, which produces air bubbles in the brazing material to be fused, which lowers the degree of vacuum of the double wall container and ruptures a sealing portion, since gas is gone and a boiling phenomenon which seals the exhaust hole by fusing it occurs in a blazing process of the brazing material in the vacuum heating furnace.
The method as described hereinabove has further problems in that organic matter, a contaminant, and hazardous gas mixed while the brazing material is blazed, flows into the space portion and greatly deteriorates adiabatic efficiency, since the brazing material is put and melted in the exhaust hole.